Multiphysics characterization of GaN HEMTs via micro-Raman spectroscopy

Abstract

Microelectronic devices based on solid-state transistor technology are a key innovation that have transformed modern society and affected many aspects of our daily lives. As we continue to increase the density and functionality of transistors, progress is limited by the intrinsic material properties of the most common semiconductor, silicon (Si). Therefore, there is an increasing need for compound semiconductor technologies with more favorable material properties, such as gallium nitride (GaN) high electron mobility transistors (HEMTs), which can operate at significantly higher voltages, current densities, and power densities than Si-based field effect transistors of the same size. However, these more strenuous operating conditions combined with the desire to operate GaN HEMTs in harsher environmental conditions lead to elevated channel temperatures, reduced device performance, and premature device failure. Thus, there is a great need to develop modeling and experimental approaches to characterize the temperature, structural evolution, and electrical performance of GaN HEMTs with microscale and even nanoscale spatial resolution. This thesis explores the application of micro-Raman spectroscopy to experimental characterization of temperature, stress, strain, and electric field in GaN HEMTs with ~1 pim spatial and ~30 ns temporal resolution, respectively. Although micro-Raman spectroscopy has been one of the most common experimental techniques for measuring temperature and stress in GaN HEMTs for the last fifteen years, many of the previous works in the field have been empirical and unable to satisfactorily explain basic features of the Raman response of HEMTs under bias. This thesis demonstrates for the first time the correct electric field dependence of the optical phonon frequencies of wurtzite GaN and measurement of the electric field along the c-axis of the GaN buffer in HEMTs biased in the pinched OFF state. With this holistic understanding of the phonon frequency dependence on temperature, stress, electric field, and strain, a methodology for simultaneously measuring temperature, stress, and electric field using the shift of three Raman peaks has been developed. Theoretical and experimental characterization of the fundamental transient thermal response of GaN HEMTs is also presented using time-resolved micro-Raman thermometry. The novel developments in this thesis represent a new "multiphysics" approach to microscale characterization of semiconductor devices, which we anticipate to have a significant impact in developing a more mechanistic and physics-based approach to transistor reliability rather than relying merely upon the statistics of a population of devices. Such an approach, we believe, will enable new semiconductor devices with unprecedented reliability and performance.